شبیه سازی روانگرایی خاک غیر اشباع با استفاده از مدل خاک در حالت بحران Simulation of liquefaction of unsaturated soil using critical state soil model

1. INTRODUCTION Soils are often subjected to cyclic loading under unsaturated conditions in practical situations, such as the deformation of embankments and reclaimed areas during an earthquake. In Japan, the Sanriku–Minami earthquake triggered a landslide in the town of Tsukidate on May 26, 2003. An artificial fill in this disaster area lost its effective confining stress under cyclic loading although the degree of saturation was around 70% [1]. The landfills along the northeastern shorelines of Tokyo Bay were liquefied because of the Great East Japan Earthquake in 2011, which caused soil subsidence in an area of around 42 km2 [2]. Thus far, several questions have been raised about the liquefaction potential of unsaturated soils. Recently, many researchers have investigated the cyclic behavior of unsaturated soils by conducting laboratory tests under fully undrained cyclic loading conditions. Ishihara et al. [3] studied the effects of the relative density and degree of saturation on the undrained behavior of nearly saturated sand through multiple series of monotonic and cyclic triaxial tests. Selim and Burak [4] conducted a torsional shear test on unsaturated silty clay to explore the small and large strain behavior of unsaturated soils. Okamura and Noguchi [5] observed the influence of air and suction pressure on the liquefactionresistance of unsaturated soils through a series of cyclic triaxial tests on a fine, clean sand and a nonplastic silt under fully undrained conditions. Liu and Xu [6] studied the effects of the degree of saturation, relative density, and confining pressure on the cyclic behavior of saturated and unsaturated sand by conducting a series of strain-controlled cyclic triaxial tests under fully undrained conditions. Tsukamoto et al. [7] conducted a series of undrained stress-controlled cyclic triaxial tests on unsaturated sand to examine the changes in the cyclic resistance of silty sand with different grain compositions. Further, Unno et al. [1, 8, 9] conducted a series of strain-controlled cyclic triaxial tests on unsaturated soils under fully undrained conditions to study the liquefaction behavior of such soils. Importantly, the existing experimental studies have revealed that the mean effective stress of an unsaturated soil having a relatively high degree of saturation gradually decreases and such a soil can be finally liquefied in a manner similar to saturated soils. So as to predict the behavior of unsaturated soils, the coupling of mechanical and hydraulic behaviors needs to be considered. Over the past decade, many researchers have proposed the coupling of an elastoplastic constitutive model and a water retention curve model to capture the behavior of unsaturated soils. Khalili et al. [10] proposed coupled flow and deformation models based on the effective stress concept for cyclic analysis using the bounding surface plasticity, the hydraulic hysteresis considering the change in density, and the coupled effect of suction hardening. Yang et al.